LED Surge Current?

[aaron]

Hi guys, been a while.

So I'll try cut a long story short. Or not...

A few months back I bought a 30 RGB LED Strip PCB. I've been running each channel off a TIP-120 Transistor, controlled by the PWM output of an Arduino, and I programmed a C# application so I'd get all sorts of colourful pulsed alerts for things like email/windows alerts and music rainbow pulsing stuff while playing audio. That was really cool.

Anyway, I had this thing running off a 2A Molex power supply I had lying around, it was one of those ones for powering an external HDD, came with the IDE to USB thing as well. Today I made some modifications to it, I added a gel battery, outdoor solar panel with built in trickle charge and a spdt relay so it can basically turn on if mains fails by itself, and stores the 'on' state in eeprom.

I've put it all up on the ceiling, and stupidly didn't even measure how much current this thing draws and just assumed 2A was plenty. I had an issue where if I simulated a power failure by unplugging the mains and then plugging it back in, when the relay coil activated and the Arduino turned on, which also instantly turned on the LED strip, the coil couldn't sustain its magnetic field I guess? Anyway, it cut out, and then basically loops continuously trying to turn on and loosing power.

I wanted to know if leds can have a large surge current, or am I borderline on the power supply? I fixed the issue perfectly by adding a 2 seconds delay before the Arduino activates the lights, but I don't know if it's safe. I'd rather not pull the strip down to measure the current. I've found that some super bright white leds can draw 20ma, which I think is the most power hungry led variant, and this has 30r, 30g and 30b. Assuming they took 20ma each that's 1.8a, the relay coil takes 200ma. So... That's conveniently 2A. 

Don't know what to do. Pull it down and test it? Do leds even have a surge current? I doubt each led on the strip would draw 60ma max? Can a small wall transformer be dangerous if it's overloaded?

Thanks 

[Marco]

What kind of voltage are you using? Does the LED strip have current limiting resistors build in?

[aaron]

It's all running off 12v. The strip has it's own cap and resistors all built in, so basically I just supply each channel 12v for white light. I've been running it solid since I got it but only today made the above changes. 

[jmoreland79]

It's tough to say what's going on since you didn't provide a schematic and some other details, but LEDs can certainly have a current spike when first powered on.

So it is possible that switching on the LEDs and Andruino simultaneously is loading down the power supply and creating a startup/fail loop.  But without knowing the specifications of the LED or if there is any current limiting parts of the circuit, that's just a guess at this point.

As for the wall transformer, it's never a good idea to overload any electrical device.  Particularly if it's a cheap simple device because it likely has little or no protection against such a condition.

[Ribster]

Most likely the sudden current draw is too much for the battery.
Batteries have internal resistance, thus when loaded with a lot of current this resistance causes a voltage drop.
I assume that current draw causes your voltage to droop too low for the relay / uC.

It's not quite clear when you have this on/off switching, is it when it's battery powered or is it when it's on your AC/DC power supply?

Without a schematic it's hard to know for sure.

[Thor-Arne]

There is a cap on the led strip? Never seen that before.

Not sure about led surge current, but the cap needs to be charged so that causes a current surge. How big this surge is depends on the size of the cap.

Having a cap on there doesn't seem right if you are going to pulse it (pwm?).

[aaron]

Oops sorry guys. There's no caps on the strip, just a bunch of surface mount resistors. My bad.
To clarify, this is when switching from the battery to mains, when mains resumes. Mains powers the coil, when mains fails the contacts go NC and the battery takes over just fine. So yeah. Feeling good this morning though, might make the effort to take the lights down and see how much current they draw to be on the safe side. 

Edit: Looks like it's 650ma for the entire panel, outputting full rgb white.

[mrkev]

Those LED strips have usually triple 5050 LEDs, 3 of them per one 5cm long segment. So You have 9 diodes on one 5cm segment.
On one diode, you'll have from 2,4V to 3,3V (deppending on collor), so they actually connect three of same color in series with appropriet resistor (those resistors are different for each color).
And yes, there is 20mA current going through each color in each segment. Sou You'll get 60mA per segment, cca 1,2A per 1m.
Thing with these LED strips is, that they design them to be OK for cars, so full brightness (and 20mA current) is in fact usually at 13V+, not 12V...
I think that your problem is elsewhere.

[CoilKid]

I have never had a problem with an LED circuit like that, but I attach a CVCC to most of the stuff I want to control the current/voltage on. I'm currently working on a solenoid project, and am using a CVCC attached to a battery as my power source. Looking at what you're doing, the same model CVCC may be useful here.

Just make sure the LED strip and the power supply fit within the product parameters.

The CVCC I'm using. Just so you know, it does not have a datasheet, but the seller has a spec list down at the bottom of the page.

I hope this helps, and good luck with your project.